The present invention relates generally to improvements in side-looking synthetic aperture imaging radar systems wherein the dispersed radar echo data is processed into images in the analog domain.
Radar imaging using side-looking synthetic aperture radar techniques has been, and continues to be, the only viable means of achieving high resolution imagery through planetary atmospheric cloud covers. Scientific applications of these radar techniques that are of immediate interest include Venus surface mapping as well as global coverage of the Earth's land masses and oceans. For such applications the digital data processing required to produce true imagery data on board a spacecraft or airborne radar platform appears impractical from the standpoint of cost, complexity, power, weight and size. The prior art approach is to utilize sophisticated ground-based digital and/or optical systems to perform the complex range and azimuth correlations required for transforming the dispersed radar instrument output data into image resolution elements. As a consequence, only minimal processing is accomplished on board the spacecraft. Since no means is yet known for compressing uncorrelated radar echo data (other than presumming and time expansion), vast quantities of "raw" data must be transmitted to Earth for processing. This typically requires reliable, high-speed (1 to 50 mbps record rates) and high capacity (greater than 1 .times. 10.sup.9 bits) tape recorders for storage of this data. The data rate and volume problem becomes even more acute when multiple-looks are required. In multiple-look systems, the data rate and volume (if the data is not processed to image form) increases directly as the number of looks.
On-board radar data processing for side-looking synthetic aperture imaging radar systems have been limited to techniques in the digital domain. Following pulse sampling, a high speed analog-to-digital converter is utilized for digitizing the samples. After digitization, the only additional operation that the prior art has found to be practical to accomplish on-board for most applications is presumming. The requirements for digitally carrying out the range and azimuth correlations on board the airborne radar platform becomes overwhelming in terms of speed and complexity. For example, just to store the digital words necessary to carry out one correlation in azimuth would typically require a high-speed, solid-state memory having a capacity in excess of 1 .times. 10.sup.7 bits. The logic operations would have to be accomplished at nanosecond rates, reflecting the need for extremely high power. Although digital processors for processing the radar image have been built, they have been totally impractical for spacecraft use from the standpoint of size, weight, power and cost. The prior art approach to airborne radar platforms is therefore, as noted above, extremely limited.
FIG. 1 illustrates a typical prior art single-look synthetic aperture radar system having a range resolution of 25 meters with a range swath of one hundred kilometers, and an azimuth resolution of 25 meters. The bandwidth of the received signal is 16 MHz. The 16 MHz return signal at processor input 11 is sampled at the Nyquist rate by a bi-polar sampler 13 that provides 32 msps at its output 15. These samples are digitized in a four-bit analog-to-digital converter 17 to a 128 mbps signal at its output 19. This 128 mbps signal is supplied to a first time expander 21 that provides a 56 mbps signal at its output 23. This signal is supplied to a presummer 25 having a presum factor of 4 that provides a 56 mbps signal at its output 27. A second time expander 31 consequently provides a 14 mbps signal at its output 33 to a tape recorder 35 having a 10.sup.9 bit capacity. The output of the recorder 35 is NRZ data at 37 which would be transmitted to Earth for processing. For a radar platform velocity of 7500 meters per second the azimuth swath length per orbit would be limited to about 525 kilometers. The recorder would be loaded in about 70 seconds. It may be feasible to help this problem by one-bit quantization. However, the potential degradation in quality due to the total loss of amplitude information makes one-bit quantization very questionable. The four bits utilized here appears to be the practical minimum for most imagery applications. Besides this severe limitation in the azimuth swath length, the extremely high digital rates utilized require high-speed logic and numerous logic functions to accomplish the sampling and quantization functions prior to the presumming operation. Although major logic systems are required to perform these functions, the ultimate information extracted therefrom still does not provide an image or useful data from which information could be extracted. Also a 10.sup.9 bit capacity tape recorder has been assumed. Spacecraft tape recorders having this capacity and the capability of recording reliably at a rate of 4 mbps over an extremely long period of time are possible but difficult to obtain. Besides the above problems, the system of FIG. 1 does not provide for a multiple-look capability. A multiple-look capability is a requisite for a clear, speckle-free image.
Assuming that a multiple-look capability of four is to be implemented by the prior art approach, the system illustrated in FIG. 2 would be representative. Thus, the 16 MHz received signal is supplied at the input 39 to bi-polar sampler 41 that produces at the Nyquist rate, 32 msps at output 43. A four-bit analog-to-digital converter 45 quantizes the samples into a 128 mbps signal at output 47. The time expander 49 reduces this rate of 56 mbps at output 51. Since a four look capability is required, the 56 mbps signal is recorded by a 10.sup.9 bit capacity tape recorder 53. This data rate to the recorder is increased by a factor of four over the system in FIG. 1 and thereby because of the radar platform velocity of 7500 meters per second, only about 18 seconds of data can be gathered on each orbit. This would be totally unexceptable for most applications. Also a reliable long-life spacecraft tape recorder having a 56 mbps record rate is probably unrealistic.